Haemodynamics determined by a genetic programme govern asymmetric development of the aortic arch

Laterality of the internal organs of vertebrates is determined by asymmetric Nodal signalling in the lateral plate mesoderm. A deficiency of such signalling results in heterotaxia syndrome, characterized by anomalous laterality of visceral organs and complex congenital heart conditions. Pitx2, the transcription factor induced by the Nodal signal, regulates left-right asymmetric morphogenesis. The cellular and molecular bases of asymmetric morphogenesis remain largely unknown, however. Here we show that ablation of unilateral Pitx2 expression in mice impairs asymmetric remodelling of the branchial arch artery (BAA) system, resulting in randomized laterality of the aortic arch. Pitx2-positive cells were found not to contribute to asymmetrically remodelled arteries. Instead, Pitx2 functions in the secondary heart field and induces a dynamic morphological change in the outflow tract of the heart, which results in the provision of an asymmetric blood supply to the sixth BAA. This uneven distribution of blood flow results in differential signalling by both the platelet-derived growth factor receptor and vascular endothelial growth factor receptor 2. The consequent stabilization of the left sixth BAA and regression of its right counterpart underlie left-sided formation of the aortic arch. Our results therefore indicate that haemodynamics, generated by a Pitx2-induced morphological change in the outflow tract, is responsible for the asymmetric remodelling of the great arteries.